This invention concerns a ring gauge for a pneumatic apparatus for measuring the outer diameter of a part, such ring gauge including an entry zone followed by a measuring zone defined by a guide cylinder in which an air jet measuring system is installed.
FIG. 1 shows very schematically in lengthwise cross-section an example of a measuring head 1 which is currently employed in this type of apparatus when it is simply a matter of measuring the diameter of a cylindrical bore of a part P.
The measuring head 1 includes an entry zone formed by a spherical portion 3 followed by a tapered part 4 and a measuring zone defined by a guide cylinder 5 in which there is installed an air jet measuring system which here includes jet nozzles 6 coupled to a source of compressed air not shown. Such measuring principle is known and has been described for instance in U.S. Pat. 4,538,449. It will be simply recalled here that it is sufficient to measure the air discharge coming out of the nozzles or the pressure of such air or indeed its flow rate in order to determine the value of the bore diameter. Naturally, for this it will also be necessary that the measuring apparatus be calibrated.
The measuring head thus briefly described is always chosen as a function of the bore of the part P to be measured in a manner such that the distance between portion 5 of such head and the internal surface of the part P does not exceed some tens of microns. The principal reason for operating thus is connected to the physical principle employed. Another reason is that very often the portion 5 is the sole guide means which one has available for the displacement and correct positioning of the head at the interior of the bore and to assure that the measures effected are correct.
It has just been mentioned that a very small play exists between the part to be measured P and the measuring head 1 which must be introduced therein, this necessitating that the head be introduced coaxially into the part if one wishes to avoid jamming. If such measurement is performed by hand on a large number of parts, the alignment which must be obtained between the head and the parts will slow down the measuring rate and thus the number of pieces which one will be able to measure per unit of time.
To overcome this difficulty the measuring head shown on FIG. 1 exhibits a special entry zone obtained by equipping the head with a spherical portion 3 followed by a tapered portion 4. The radius of sphere 3 is chosen to be equal to the radius of the cylinder 5. Thus head 1 may contact the part P at an angle of substantial misalignment, for instance 15.degree. as is shown on FIG. 1. The head 1 may be introduced into part P until the edge 7 of bore 8 comes into contact with cone 4. In continuing the advancing movement head 7 will slide along the cone 4 until the cylindrical portion 5 of the head in turn penetrates into the bore. At this moment axis 9 of the measuring head will be coaxial with axis 10 of the part P and the head may be entirely introduced into the interior of the bore.
This manner of forming the measuring head is known and is applied for measuring the interior diameter of parts. We will now examine the opposite of this arrangement which consists of measuring by the same method the outer diameter of a cylindrical part. For this reference will be made to FIG. 2.
The measuring head 20 then becomes a ring which includes a tubular portion 21. The measuring zone is here constituted by a guide cylinder 22 in which are installed the jet nozzles 23 for compressed air. As in the preceding case, the diameter of the guide cylinder is arranged to be slightly greater than the diameter of the part to the measured P' (some tens of microns).
At an intermediate stage in the development of the present invention I invisioned a modification of the gauge of FIG. 1 to permit the measurement of the external diameter of a cylindrical body. This modification is illustrated in FIG. 2. To avoid jamming the part to be measured, ed to apply to the ring gauge the same entry arrangement as that which has been described for the sleeve of FIG. 1. There is found at the entry to the ring a toroidal region 24 followed by a conical region 25. The most constricted internal diameter of the torus is made equal to the diameter of the guide cylinder 22. It will however immediately be noted that a part to be measured P' introduced at a certain angle into the ring (here some 15.degree.) will butt against the toroidal portion 24 by its edge 28 and it will be necessary to align the part P' in order that penetration may take place.
It will be observed that such jamming will also be produced for less unfavourable cases than that which has been shown on FIG. 2. Since the most constricted diameter of the torus is slightly greater than the diameter of the part to be measured P', this part may pass the constricted diameter if the angle of incidence formed by axis 26 of the ring and the axis 27 of the part P' is small. After this however, jamming may arise analogous to that which is known from a conical shank of the morse taper. In conclusion, the entry system proposed for a plug penetrating a bore is not suitable for a ring into which must be introduced a cylinder.